On startup and before we connected to a tablet, let's display a missing tablet
icon. Taken from the tablet icon, combined with the X from one of the network
icons in the Adwaita theme.
Once we get a sync notification, we just hide that one and done.
Drop the user-visible separation, make everything just Tuhi. Directory
structure remains largely separate (tuhi vs tuhigui) except for the po files
which are in the top-level directory because convention.
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>